TVS breakdown voltages for higher currents

[Joerg]

Hello Folks,

Does anyone know a manufacturer that provides a wee bit more info in
datasheets for their TVS?

The issue:

We have SMAJ78CA in a board because long cables, filters and such can
cause nasty ringing. The board must tolerate several 100msec bursts of
80V amplitude in a row, but only on rare occasions and then such a burst
is a one-time event. Long story short, these things won't clamp well
before 100V is reached in the ringing and then some stuff with 100V abs
max ratings can go phhhhut.

Unfortunately min-max breakdown voltage for such TVS is only spec'd at
1mA which isn't helpful. If there was a spec for, say 20mA, that would
be different. I have inquired but not much hope. Does anyone know a mfg
that specs also at higher currents? Must be in the tables and not just
typical graphs because this product goes into a safety-critical and thus
regulated market.

Tried a few SPICE models such as the ".SUBCKT 1smb70at3g" from:

http://www.onsemi.com/pub/Collateral/1SMA70AT3G.LIB

But they all seem a bit on the primitive side, either a simple diode
model or piecewise linear, and also no min-max.

 

[Don Lancaster]

Hello Folks,

Does anyone know a manufacturer that provides a wee bit more info in
datasheets for their TVS?

Littlefuse.


--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Joerg]

Littlefuse.

Unfortunately they don't :-(

http://www.diodes.com/datasheets/ds19005.pdf

 

[[email protected]]

Hello Folks,



Does anyone know a manufacturer that provides a wee bit more info in

datasheets for their TVS?



The issue:



We have SMAJ78CA in a board because long cables, filters and such can

cause nasty ringing. The board must tolerate several 100msec bursts of

80V amplitude in a row, but only on rare occasions and then such a burst

is a one-time event. Long story short, these things won't clamp well

before 100V is reached in the ringing and then some stuff with 100V abs

max ratings can go phhhhut.



Unfortunately min-max breakdown voltage for such TVS is only spec'd at

1mA which isn't helpful. If there was a spec for, say 20mA, that would

be different. I have inquired but not much hope. Does anyone know a mfg

that specs also at higher currents? Must be in the tables and not just

typical graphs because this product goes into a safety-critical and thus

regulated market.



Tried a few SPICE models such as the ".SUBCKT 1smb70at3g" from:



http://www.onsemi.com/pub/Collateral/1SMA70AT3G.LIB



But they all seem a bit on the primitive side, either a simple diode

model or piecewise linear, and also no min-max.



--

Regards, Joerg



http://www.analogconsultants.com/

Epcos has V/I characteristics of their TVS products over decades of current:
http://www.epcos.com/inf/75/db/CTVS_09/Leaded__SHCV.pdf

 

[Fred Bartoli]

Joerg a écrit :

Hello Folks,

Does anyone know a manufacturer that provides a wee bit more info in
datasheets for their TVS?

The issue:

We have SMAJ78CA in a board because long cables, filters and such can
cause nasty ringing. The board must tolerate several 100msec bursts of
80V amplitude in a row, but only on rare occasions and then such a burst
is a one-time event. Long story short, these things won't clamp well
before 100V is reached in the ringing and then some stuff with 100V abs
max ratings can go phhhhut.

Unfortunately min-max breakdown voltage for such TVS is only spec'd at
1mA which isn't helpful. If there was a spec for, say 20mA, that would
be different. I have inquired but not much hope. Does anyone know a mfg
that specs also at higher currents? Must be in the tables and not just
typical graphs because this product goes into a safety-critical and thus
regulated market.

Tried a few SPICE models such as the ".SUBCKT 1smb70at3g" from:

http://www.onsemi.com/pub/Collateral/1SMA70AT3G.LIB

But they all seem a bit on the primitive side, either a simple diode
model or piecewise linear, and also no min-max.

I doubt you'll find anything like that. Plus their tempco will give you
even more hard time.

Your mention of 20mA seems to indicate that your working impedance isn't
too low, which would help...
I had that sort of problem once and I resorted to 2 inductor-TVS stages
(had to withstand 8/20 surge levels) followed by a resistor/zener clamp.
My impedances where high (supplying CMOS logic)

In you case maybe you can do that, or a TVS clamp followed by a zener
driven voltage limiter...

 

[Joerg]

Joerg a écrit :

I doubt you'll find anything like that. Plus their tempco will give you
even more hard time.

I was able to get that for a zener once. But it was a smaller
manufacturer and they are often more flexible. They actually gave me
their QC acceptance criteria sheets and they tested at many more current
values than the one listed in the datasheet.

Your mention of 20mA seems to indicate that your working impedance isn't
too low, which would help...

Actually it is unknown and can be very low. What I mean is that if the
TVS is guaranteed to run at no more than, say, 20mA at 80V we'd be fine.
Even 50mA would be ok. This is because those spikes will be short and
seldom.

Probably that is the case for a TVS with 70V standoff voltage or
possibly even for lower ones. But I need hard data for this design.

I had that sort of problem once and I resorted to 2 inductor-TVS stages
(had to withstand 8/20 surge levels) followed by a resistor/zener clamp.
My impedances where high (supplying CMOS logic)

In you case maybe you can do that, or a TVS clamp followed by a zener
driven voltage limiter...

I sure would have liked to provide an electronic clamp using a big old
FET but the problem is we did not have the required board space.

 

[Joerg]

Joerg a écrit :

I doubt you'll find anything like that. Plus their tempco will give you
even more hard time.

I was able to get that for a zener once. But it was a smaller
manufacturer and they are often more flexible. They actually gave me
their QC acceptance criteria sheets and they tested at many more current
values than the one listed in the datasheet.

Your mention of 20mA seems to indicate that your working impedance isn't
too low, which would help...

Actually it is unknown and can be very low. What I mean is that if the
TVS is guaranteed to run at no more than, say, 20mA at 80V we'd be fine.
Even 50mA would be ok. This is because those spikes will be short and
seldom.

Probably that is the case for a TVS with 70V standoff voltage or
possibly even for lower ones. But I need hard data for this design.

I had that sort of problem once and I resorted to 2 inductor-TVS stages
(had to withstand 8/20 surge levels) followed by a resistor/zener clamp.
My impedances where high (supplying CMOS logic)

In you case maybe you can do that, or a TVS clamp followed by a zener
driven voltage limiter...

I sure would have liked to provide an electronic clamp using a big old
FET but the problem is we did not have the required board space.

 

[Joerg]

I find this tool useful for converting data sheet graphs into
numbers...

http://getdata-graph-digitizer.com/

The I sometimes run that data thru a curve fitter such as...


http://www.downsoft.com/windows/windows-office-tools/windows-demonstration/wincurvefit-118-7216.html

To get an equation I can use in behavioral modeling ;-)

Yabbut ... I need guaranteed values here. Not just graphs or typical values.

 

[Joerg]

The real world has no guaranteed values. Real engineers can work with
a range.

He who works on mission-critical electronics shall be cautious about
that. You can't step in front of an NTSB inspector and say "But the
graph showed that this should normally not have happened!".


TINSTAAFL ;-)

Not "is", it's "ain't", man

 

[[email protected]]

Hello Folks,



Does anyone know a manufacturer that provides a wee bit more info in

datasheets for their TVS?



The issue:



We have SMAJ78CA in a board because long cables, filters and such can

cause nasty ringing. The board must tolerate several 100msec bursts of

80V amplitude in a row, but only on rare occasions and then such a burst

is a one-time event. Long story short, these things won't clamp well

before 100V is reached in the ringing and then some stuff with 100V abs

max ratings can go phhhhut.

Many of these flea-sized TVS have really weak average power handling capability, usually just 10s mW, so those 100msec burst times of yours have a good chance of blowing the SMAJ78CA. You better start with the repetitive ratepower handling of the part before you waste a bunch of time looking for V/I limit guarantees.

 

[Joerg]

Many of these flea-sized TVS have really weak average power handling
capability, usually just 10s mW, so those 100msec burst times of
yours have a good chance of blowing the SMAJ78CA. You better start
with the repetitive rate power handling of the part before you waste
a bunch of time looking for V/I limit guarantees.

SMA isn't exactly flea-sized. They can take a watt or two for a few
hundred msec. That's all I am asking them to do.

[...]

 

[Joerg]

Clamping is the brute-force way to protect your gear. You're
volunteering to absorb all the energy your enemy can throw at you. A
more zen-like strategy is to step aside, namely limit the input
current before you try to clamp it.

Zen wasn't possible, I had about the square-footage of a fly to clamp at
two places, no chance to have opening devices because they'd have to be
quite major.

Anyhow, there will never be much energy above 80V and all I want is
something that is guaranteed not to conduct for than a few tens of
milliamps at 80V. The normal voltage range is less than 32V and the 80V
peaks will be very short and a rare occurrence. I am fairly sure a 70V
TVS can do that but a bench test isn't good enough here.

 

[[email protected]]

Many of these flea-sized TVS have really weak average power handling

capability, usually just 10s mW, so those 100msec burst times of

yours have a good chance of blowing the SMAJ78CA. You better start

with the repetitive rate power handling of the part before you waste

a bunch of time looking for V/I limit guarantees.

SMA isn't exactly flea-sized. They can take a watt or two for a few

hundred msec. That's all I am asking them to do.



[...]

No they can't. The Vishay ds says repetitive power handling of 300W at 0.01% duty which comes to 30mW.

 

[Joerg]

On Saturday, September 22, 2012 10:47:06 AM UTC-4, Joerg wrote:
Hello Folks,
Does anyone know a manufacturer that provides a wee bit more info
in
datasheets for their TVS?
The issue:
We have SMAJ78CA in a board because long cables, filters and such
can
cause nasty ringing. The board must tolerate several 100msec bursts
of
80V amplitude in a row, but only on rare occasions and then such a
burst
is a one-time event. Long story short, these things won't clamp
well
before 100V is reached in the ringing and then some stuff with 100V
abs
max ratings can go phhhhut.
Many of these flea-sized TVS have really weak average power handling
capability, usually just 10s mW, so those 100msec burst times of
yours have a good chance of blowing the SMAJ78CA. You better start
with the repetitive rate power handling of the part before you waste
a bunch of time looking for V/I limit guarantees.

SMA isn't exactly flea-sized. They can take a watt or two for a few

hundred msec. That's all I am asking them to do.



[...]

No they can't. The Vishay ds says repetitive power handling of 300W at 0.01% duty which comes to 30mW.

Of course not continuously. That's why I wrote "for a few hundred msec".
As mentioned before, the duty cycle will be extremely low, and it'll be
less than 0.01%.

I am not going to sink 300W pulse into them but maybe 1-2W pulses.

 

[[email protected]]

[email protected] wrote:

On Saturday, September 22, 2012 10:47:06 AM UTC-4, Joerg wrote:
Hello Folks,
Does anyone know a manufacturer that provides a wee bit more info
in
datasheets for their TVS?
The issue:
We have SMAJ78CA in a board because long cables, filters and such
can
cause nasty ringing. The board must tolerate several 100msec bursts
of
80V amplitude in a row, but only on rare occasions and then such a
burst
is a one-time event. Long story short, these things won't clamp
well
before 100V is reached in the ringing and then some stuff with 100V
abs
max ratings can go phhhhut.
Many of these flea-sized TVS have really weak average power handling
capability, usually just 10s mW, so those 100msec burst times of
yours have a good chance of blowing the SMAJ78CA. You better start
with the repetitive rate power handling of the part before you waste
a bunch of time looking for V/I limit guarantees.


SMA isn't exactly flea-sized. They can take a watt or two for a few

hundred msec. That's all I am asking them to do.



[...]

No they can't. The Vishay ds says repetitive power handling of 300W at 0.01% duty which comes to 30mW.

Of course not continuously. That's why I wrote "for a few hundred msec".

As mentioned before, the duty cycle will be extremely low, and it'll be

less than 0.01%.



I am not going to sink 300W pulse into them but maybe 1-2W pulses.



--

Regards, Joerg



http://www.analogconsultants.com/

Okay , from Fig 5. http://www.mouser.com/catalog/specsheets/smaj50.pdf the transient thermal impedance is less than 10oC/W (~7) for the 100ms-1000ms pulses, so it's a non-issue. But anything approaching continuous and Rja goes towards 120oC/W in that flea package.

 

[Robert Baer]

Hello Folks,

Does anyone know a manufacturer that provides a wee bit more info in
datasheets for their TVS?

The issue:

We have SMAJ78CA in a board because long cables, filters and such can
cause nasty ringing. The board must tolerate several 100msec bursts of
80V amplitude in a row, but only on rare occasions and then such a burst
is a one-time event. Long story short, these things won't clamp well
before 100V is reached in the ringing and then some stuff with 100V abs
max ratings can go phhhhut.

Unfortunately min-max breakdown voltage for such TVS is only spec'd at
1mA which isn't helpful. If there was a spec for, say 20mA, that would
be different. I have inquired but not much hope. Does anyone know a mfg
that specs also at higher currents? Must be in the tables and not just
typical graphs because this product goes into a safety-critical and thus
regulated market.

Tried a few SPICE models such as the ".SUBCKT 1smb70at3g" from:

http://www.onsemi.com/pub/Collateral/1SMA70AT3G.LIB

But they all seem a bit on the primitive side, either a simple diode
model or piecewise linear, and also no min-max.

As i vaguely remember, a TVS is a crappy limiter, with a slow
limiting slope and high incremental resistance reminiscent of that seen
for a 2.7V or 3.3V zener or like that of a forward biased diode.
Use a real zener with a current limiting resistor in series so that
it will not blow out.

 

[Joerg]

Hello Folks, Does anyone know a manufacturer that provides
a wee bit more info in datasheets for their TVS? The issue:
We have SMAJ78CA in a board because long cables, filters
and such can cause nasty ringing. The board must tolerate
several 100msec bursts of 80V amplitude in a row, but only
on rare occasions and then such a burst is a one-time
event. Long story short, these things won't clamp well
before 100V is reached in the ringing and then some stuff
with 100V abs max ratings can go phhhhut.
Many of these flea-sized TVS have really weak average power
handling capability, usually just 10s mW, so those 100msec
burst times of yours have a good chance of blowing the
SMAJ78CA. You better start with the repetitive rate power
handling of the part before you waste a bunch of time looking
for V/I limit guarantees.
SMA isn't exactly flea-sized. They can take a watt or two for a
few hundred msec. That's all I am asking them to do. [...]
No they can't. The Vishay ds says repetitive power handling of
300W at 0.01% duty which comes to 30mW.

Of course not continuously. That's why I wrote "for a few hundred
msec".

As mentioned before, the duty cycle will be extremely low, and
it'll be

less than 0.01%.



I am not going to sink 300W pulse into them but maybe 1-2W pulses.



--

Regards, Joerg



http://www.analogconsultants.com/

Okay , from Fig 5.
http://www.mouser.com/catalog/specsheets/smaj50.pdf the transient
thermal impedance is less than 10oC/W (~7) for the 100ms-1000ms
pulses, so it's a non-issue. But anything approaching continuous and
Rja goes towards 120oC/W in that flea package.

That is exactly what I meant. It is a non-issue _if_ you know how much
it will conduct at 80V, worst case maximum. And I don't, hence my
inquiry and my post here.

 

[Joerg]

As i vaguely remember, a TVS is a crappy limiter, with a slow limiting
slope and high incremental resistance reminiscent of that seen for a
2.7V or 3.3V zener or like that of a forward biased diode.
Use a real zener with a current limiting resistor in series so that it
will not blow out.

Zeners would be (somewhat) better but, for example, a 82V zener in an
SMA package can only take 55mA of surge current and only for less than
10msec non-repetitive:

http://www.vishay.com/doc?85782

 

[Nemo]

We have SMAJ78CA in a board because long cables, filters and such can

cause nasty ringing. The board must tolerate several 100msec bursts of
80V amplitude in a row, but only on rare occasions and then such a burst
is a one-time event. Long story short, these things won't clamp well
before 100V is reached in the ringing and then some stuff with 100V abs
max ratings can go phhhhut.

Unfortunately min-max breakdown voltage for such TVS is only spec'd at
1mA which isn't helpful. If there was a spec for, say 20mA, that would
be different.

You could try a gas discharge tube as a first line of defence, there are
some tiny surface mount ones around now which trip around 70V, and then
clamp around 20V. Can take amps for much longer than a TVS.

 

[Martin Riddle]

Zeners would be (somewhat) better but, for example, a 82V zener in an
SMA package can only take 55mA of surge current and only for less than
10msec non-repetitive:

http://www.vishay.com/doc?85782

Gas discharge?
<http://www.mouser.com/ProductDetail...=sGAEpiMZZMsdWQmBKEIh8nxzCboN3V1ERArz/wH/83A=>

Cheers